Transition between direct gap and indirect gap in two dimensional hydrogenated honeycomb SixGe1-x alloys

Using first-principles calculations, we have explored the structural and electronic properties of fully hydrogenated honeycomb SixGe1-x H alloys. Finite band gaps are opened by hydrogenation for x in the whole range from 0 to 1, while their nature and values can be tuned by x. When x is <0.7, the band gap is direct (from Gamma to Gamma). And when x is >= 0.7, the gap turns into indirect (from Gamma to M). For all the computed compositions, the two kinds of energy differences between valence band and conduction band, Gamma-Gamma and Gamma-M, are described well by two polynomial functions of x. The smaller of the two functions gives a good prediction for the overall band gap at any x. The two curves cross at x = 0.7, leading to the change of band gap type. At PBE level, the values of band gap for different x spread from 1.09 to 2.29 eV. These findings give a new route to tune the electronic properties of these materials and may have potential applications in nanoscale optoelectronics.